The invention relates to a method and apparatus for recovering resources from below the earth's surface, particularly heat, and particularly geothermal energy resources. It is generally recognized by geologists that within a distance of 20 miles beneath any point of the earth's surface, including the ocean floors, temperatures reach levels that would usefully power heat engines. The only previous barrier to the recovery of such energy resources was the difficulty of providing adequate heat-exchanging means at depths with substantial geothermal resources. Once that barrier is overcome at a certain point of the earth's surface that point becomes a suitable site for geothermal energy recovery operations. Geologists believe that the easily available geothermal resources which have been successfully recovered in the past existed close to the earth's surface as the result of natural heat-exchanging mechanisms at greater depths. These natural heat-exchanging formations make available only a minute fraction of the potential resources lying within 20 miles of the surface. The earth's total geothermal resources greatly exceed the world's energy needs, not only for today but for the foreseeable future. These resources have been almost totally inaccessible to past methods and apparatus, not because the drilling apparatus had not reached a high state of development, but because of the inherent limitations, both technological and economic, of the past drilling methods. The present invention overcomes all of the limitations inherent in the prior methods. It is the purpose of the present invention to make the geothermal resources of the earth generally accessible to meet the world's great need for energy, particularly for non-polluting energy, which is an essential requirement of life and prosperity.
In the past it has been proposed to drill two separate holes into a solid rock formation with heat-drills or other drills, fracture the rock hydraulically or by nuclear explosions to create connecting passages for fluid flow, and then circulate fluid down one hole, through the connecting passages and up the other hole to recover heat. In the case of nuclear explosions part of the heat generated by the nuclear reaction would also be recoverable. It has also been proposed to drill one hole, remove the drill, and insert pipes into the hole through which a fluid would circulate to remove heat (see U.S. Pat. Nos. 3,274,769; 3,470,943; and 3,521,699). Various types of apparatus have been proposed for doing the drilling, such as the heat-drills disclosed in U.S. Pat. Nos. 3,396,806; 3,357,505, and 3,693,731. While prior methods and apparatus are in general satisfactory, they generally have one or more of the following drawbacks: requiring separate drilling operations for each hole; requiring drilling pipe of a length on the order of the depth of the hole to circulate drilling mud to remove excess rock; requiring considerable mechanical force to be applied to the drill as an essential part of the drilling operation; causing considerable abrasion of the drill which tends to wear down the radial dimension of the drill and reduce the cross-sectional dimensions of the hole; requiring repeated casings to be positioned each extending from the surface to the vicinity of the hole botton at the time to prevent collapse of the shaft walls; requiring each casing to fit inside the smallest part of the hole it can reach in a hole of decreasing diameter; requiring each casing to fit inside the ones already in place and hence be of successively smaller diameter; requiring repeated recovery and replacement of the worn drills; requiring each drill used to be able to fit in the smallest part of the shaft at the bottom and hence be smaller than the last; requiring each successive drill to fit in the casings in place and hence be smaller for each new casing; not usefully recovering any of the energy expended in the drilling operation; not usefully recovering the energy resources drilled for until after the completion of the drilling process; needing separate operations in addition to drilling to be performed to perforate the casings before circulation of fluids between the shaft and the surrounding formations begins; requiring separate operations in addition to drilling to be performed to the casings to prepare for fracturing operations on the surrounding rock; having no suitable means in heat-drilling operations of controlling the position or orientation of fractures which may be desirable and avoiding fractures which may be undesirable; having no suitable means of connecting two separate holes in solid rock other than fracturing the rock to form connecting passages from hole to hole; having no suitable means to extend a producing two-hole geothermal well to a greater depth without sealing the connecting passages between the holes; requiring heat-drills to apply their thrusting force to their heating surfaces to move molten rock and the drill; requiring heat-drills to apply such force to their heating surfaces that employable temperatures are limited by the structural strength of the materials rather than by the materials' stability at higher temperatures. The method and apparatus of the present invention overcomes all of the above mentioned drawbacks.
According to the teachings of the present invention, a drill body having a particular shape with a heating element of a particular configuration attached thereto is used to drill into the earth and from two shafts at the same time in the earth. The two shafts are in fluid communication through the body of the drill and are used to circulate a "drilling mud" through the drill body to carry off excess rock. The heating element operates at a temperature well above the melting point of the rock, melts through rock ahead of the drill body, and raises the rock through which it passes to well above the rock's melting point, raising the average temperature of the rock through which the drill body passes, however, to a selected lower degree above the rock's melting point. The heating element passes through a plurality of rock portions spaced throughout the region to be melted, and sweeps through only a fraction of the spatial volume swept out by the drill body. The molten rock takes one of two alternate paths: it either flows into the interior of the drill body and thence into the drilling mud circulating through the drill body and thence to the surface; or it flows around the exterior of the drill body to the top of the drill body, which makes two shafts in the molten rock. Means are provided for making the shafts the desired shape, for the gradual cooling of the shaft walls, and for the maintenance of the molten rock in the desired shape until the rock solidifies leaving two permanent shafts. Means are also provided for causing the shafts formed to spiral around one another in a controlled manner.
The walls of the downflow shaft are formed with one or more grooves extending throughout the length of the shaft, shaped like a V cut into the wall. The grooves are made to facilitate later fracturing of the surrounding rock, and a concentration of thermal stress at the apex of the V cut also facilitates later fracturing and may even cause initial fractures to form at the apex by thermal stress. The drilling mud circulating through the drill body and the shafts absorbs heat in its passage and that heat may be utilized while drilling is taking place; in particular a well-known effect due to absorbed heat is the "thermosyphonic effect" which creates a driving force acting on the fluid in its direction of motion and which may in some cases be the only pumping force needed to circulate the fluid. A heat exchanger at the surface removes heat from the drilling mud for any desired use, particularly to help provide energy to the drilling operation.
Once the drill has reached the desired depth, fractures extending into the surrounding rock are introduced through the grooves hydaulically, propped open by well-known means, and partitioned by material forced horizontally into the fractures at selected depths to form heat-collecting cells. If the shafts spiral around one another the fracture surfaces radiating from the downflow shaft will spiral in the same direction and rate as the shafts, and will resemble helical surfaces. The drilling mud, or a fluid which replaces the drilling mud, will collect heat from the surrounding rock as the fluid circulates through the shafts, the fracture cells, and the drill body. As the drilling progresses the "thermosyphonic effect" provides a pumping action to the circulating fluid which will increase to the point that a turbine may need to be placed in the flow stream to limit the rate of flow. The turbine may be placed in either the downflow shaft or the upflow stream, but the downflow does not contain the rock being removed and may be preferred. The turbine may also be used to provide power. The drill may be re-started at a later date to proceed to a lower level without any need to seal off the fractures. Fractures extending from one shaft do not form part of connecting passages to the other shaft, so all fluid circulating from one shaft to the other passes through the drill body.
It is the principal object of this invention to recover geothermal heat from the earth in a manner which may be more widely applied to a greater variety of geological formations than current methods. It is a further object of this invention to provide a method of drilling which can drill to greater depths than drills in the past. It is a further object of this invention to provide mans to study regions within the earth including those under ice, which have been known only by theory before, and whose resources are thus largely a matter of speculation. It may for example be possible that the oil and gas resources currently found near the surface trapped in sedimentary rock formations are deposits of chemicals migrating through the basement rock from greater depths, and fluids circulating through fractures in the solid rock at sufficient depth may absorb useful amounts of such petrochemicals; thus exploration of unknown regions for unknown resources is a further object of this invention. These and other objects of the present invention will become clear upon an inspection of the detailed description of the invention and from the appended claims.